1. Field of the Invention:
The present invention relates to electronic signal filter technology and more particularly, to a cavity filter, which has two series of resonance chambers bilaterally connected between an antenna port and two opposing signal input/output ports in a resonant space to provide a cross-coupling feedback, getting better stop-band flatness and improving the quality of the signal received by the signal receiver using the cavity filter.
2. Description of the Related Art:
Following fast development of communication technology, many advanced wired and wireless signal transmitting and receiving equipment have been created and are widely used in different fields. However, due to limited wireless communication channels, full bandwidth utilization is quite important. For full bandwidth utilization, communication capacity and quality must be well improved. As different channels may be close to one another, channel isolation must be well done to prevent interference and to maintain signal transmission quality. For removing noises in a wireless communication application, a cavity filter is usually used. However, it is not easy to create a cavity that effectively removes noises and achieves excellent channel-to-channel isolation.
A regular bandpass cavity filter (duplexer) allows bi-directional communication of the energy at a particular frequency range over a single channel and attenuates the energy that is out of this particular frequency range. However, a cavity filter cannot completely isolate the stop-band energy, causing instability of transmission signal at the stop-band frequency. A signal feedback design may be employed to regulate the energy at the stop-band frequency. FIGS. 5 and 6 illustrate a cavity filter (duplexer) according to the prior art. According to this design, the cavity filter (duplexer) A defines a plurality of resonance chambers A01 in a resonant space A0 therein, a channel Al in communication between each two adjacent resonance chambers A01, an antenna port A2 at the center of the resonant space A0 for transmitting/receiving signals, and two signal input/output ports A3 at two distal ends of the resonant space A0 for signal transmission. Signals received (or transmitted) by the antenna port A2 are filtered through the resonance chambers A01 and then outputted by the signal input/output ports A3. According to this design, when a signal goes through the resonance chambers A01, attenuated signal components will be diffused to interfere with the performance of the cavity filter, affecting signal receiving or transmitting stability. As illustrated in FIG. 7, the frequency flatness of the stop-band ranges from 66 dB˜74 dB. This wide flatness range causes signal instability.
Therefore, it is desirable to provide a cavity filter (duplexer), which enhances signal receiving/transmitting stability within a predetermined receivable range.